Electronics and Communication Engineering - Power Electronics - Discussion

@All.

To understand why option 'C' is the correct answer, let's first discuss what a 12-pulse operation is. 12-Pulse Operation: In power systems, a 12-pulse operation refers to a method to reduce the harmonic distortion in the output voltage or current of a rectifier. It is achieved by combining two separate 6-pulse rectifiers with a phase shift of 30 degrees between them.
Each 6-pulse rectifier consists of six diodes arranged in a three-phase bridge configuration. The input to each rectifier is a three-phase AC supply. By connecting two such rectifiers with a phase shift of 30 degrees, the total harmonic distortion is significantly reduced.

Now, let's analyze the options given: a) Y - Y and Y - Y transformers: In a Y-Y transformer connection, the primary winding and secondary winding are both connected in a star configuration. This connection does not provide the necessary phase shift of 30 degrees required for a 12-pulse operation. Therefore, this option is incorrect.

b) Δ - Δ and Δ - Δ transformers: In a Δ-Δ transformer connection, the primary winding and secondary winding are both connected in a delta configuration. Similar to option 'a', this connection does not provide the necessary phase shift of 30 degrees required for a 12-pulse operation. Therefore, this option is also incorrect.

c) Y - Y and Y - Δ transformers: In a Y-Δ transformer connection, the primary winding is connected in a star configuration, while the secondary winding is connected in a delta configuration. This connection provides the required phase shift of 30 degrees between the two 6-pulse units, making it suitable for a 12-pulse operation. Therefore, this option is correct.

d) Y - Δ and Y - Δ transformers: In a Y-Δ transformer connection, both the primary and secondary windings are connected in a delta configuration. This connection does not provide the necessary phase shift of 30 degrees required for a 12-pulse operation.

Therefore, this option is incorrect.

Conclusion: From the analysis above, we can conclude that option 'C' (Y-Y and Y-Δ transformers) is the correct answer for a 12-pulse operation. This connection provides the required phase shift between the two 6-pulse units, resulting in reduced harmonic distortion.

I am not getting it, please elaborate it.

The phase displacement between the two AC supplies is usually 30° and is realised by using converter transformers with two different secondary windings (or valve windings). Usually one of the valve windings is a star (Y)-connected and the other is delta-connected. With twelve valves connecting each of the two sets of three phases to the two DC rails, there is a phase change every 30°, and harmonics are considerably reduced. For this reason, the twelve-pulse system has become standard on almost all line-commutated converter HVDC systems.

Please explain.